1. Field of the Invention
The invention relates to a lighting and/or signalling device comprising means for removing heat produced by the light source or sources of the device. More particularly, the invention relates to a lighting and/or signalling device for a motor vehicle comprising a chamber inside which at least one light source is positioned, the chamber having an outer wall provided with a heat exchange zone for transferring heat from the inside to the outside of the chamber. The invention also relates to a vehicle fitted with such a device, as well as a method for producing a casing adapted to the present invention.
2. Description of the Related Art
The “heat exchange zone” of the outer wall is understood to mean a zone that will exchange heat from the inside to the outside of the chamber, in a preferred manner compared with the other zones on the outer wall that will not have been defined as heat exchange zones. For example, a zone of the outer wall consisting of a thermally conducting material or a heat exchanger included in the wall constitutes a heat exchange zone.
The present invention is particularly valuable in the case of a lighting and/or signalling device comprising electroluminescent diodes or LEDs, more particularly in the case of headlights using power LEDs.
In the prior art, the use of an electroluminescent diode has been already proposed by reason of the many advantages that it presents.
Indeed, an LED consumes less electrical energy, even at equal luminous flux intensity, than a discharge or incandescent lamp that is traditionally used in the automotive field.
An LED does not radiate in an omnidirectional manner, but radiates in a more directional manner than a discharge lamp. Thus the quantity of light lost, and therefore of electrical energy lost, is lower.
LEDs also take up little room and may be positioned in much more confined spaces, and their particular shape offers new possibilities for producing and arranging complex surfaces that are associated with them.
Initially, LEDs were used in signalling lights or the rear lights of vehicles that required much less luminous power than lighting devices.
At the present time, an increase in the available power for LEDs makes it possible to envisage novel uses for these light sources, in particular for achieving lighting functions in lights at the front of the vehicle. In this case, the LEDs used are power LEDs. The expression “power LED” denotes an electroluminescent diode of which the luminous flux is of the order of at least 30 lumens.
However, a power LED produces heat as it operates. Heating of the LED is prejudicial to its satisfactory functioning, since the more the temperature of the diode rises, the more its luminous flux diminishes. Moreover, in the confined space of a headlight, the possibilities of removing heat are very limited and the temperature inside the headlight can rise very rapidly. Now, LEDs do not withstand temperatures as high as those that discharge lamps or halogen lamps withstand. More particularly, LEDs possess a maximum junction temperature of between 125 degrees Celsius (°C.) and 150° C., above which LEDs exhibit not only a reduction in efficiency but also a risk of breaking.
The heat produced by the diodes is not produced by its beam, which does not contain infrared radiation (cold light is referred to). This heat is, on the other hand, produced in the LED itself. In order to reduce its operating temperature, the LED has a metal heat-dissipating base, often called a “slug” that makes it possible to establish thermal contact with a heat dissipater, such a radiator, in order to dissipate the heat produced by the LED. Such a dissipater is in particular described in application EP-A-1 139 019. However, the heat dissipater removes heat in the region of the LED but inside the headlight, of which the internal temperature will increase.
It is difficult to evacuate heat to outside the headlight, in particular by reason of the fact that this is in contact with the engine compartment which constitutes a heat source, often between 70° C. and 80° C. when the vehicle is operating. For example, when the temperature outside the vehicle is 40° C., the temperature in the region of the engine compartment being approximately 70° C., a temperature of 90° C. is easily reached inside the headlight. Consequently, it becomes much more difficult with a single heat dissipater to remove heat in the region of the LED so that it does not reach its maximum junction temperature.
Various solutions have appeared in the prior art for removing heat from the inside to the outside of the headlight.
Document US2006/0076572 describes, and illustrates in its FIG. 4, a headlight inside which a diode is positioned mounted on a heat dissipater. A heat exchanger is positioned at the bottom of the casing of the headlight, on the wall of this casing. Through the difference in thermal gradient, heat dissipated by the heat dissipater is transferred through the space of the casing to the heat exchanger, which transmits this heat outside the casing. As illustrated in FIG. 5 of US2006/0076572, it is possible to position a fan for circulating air between the heat dissipater and the heat exchanger. Another solution illustrated in FIG. 6 of US2006/0076572, consists of directly connecting the casing to the heat dissipater on which the LED is mounted. According to another embodiment, illustrated in FIG. 7 of US2006/0076572, the heat dissipater is directly connected to a heat conductor, that in this way conducts heat to one of the parts of the heat conductor forming one with part of the wall of the casing. However, these embodiments do not enable the temperature to be reduced effectively below a certain temperature. Indeed, the heat exchange between the outside and the inside of the headlight occurs in the region of the casing and in particular of the engine compartment.
Patent application US2006/0181894 discloses a headlight of which the casing is traversed right through with a cooling duct inside which air circulates coming from outside the vehicle. Vanes positioned on the duct, inside the chamber defined by the casing closed by the outer glass, enables heat inside the chamber to be evacuated through the duct. However, the structure of such a headlight is complex to produce in the region of the wall of the casing, supplementary holes having to be sealed.
Patent application DE10258623 discloses a headlight in which the casing of the headlight is thermally insulated. In order to be insulated, the wall of the casing consists of two walls separated by an insulating space and sealed between each other at their ends, in this way forming a closed cavity. A thermally conducting part is positioned at the bottom of the casing and inside the chamber consisting of the casing and the glass. It extends from the light source in the vicinity of the glass, in this way enabling the air to be heated in the vicinity of the base of the glass of the glass of the vehicle. Since this air is hotter, it will rise inside the chamber, along the wall of the glass and thus be cooled. This type of headlight permits positioning in the region of the engine compartment while insulating the casing from the engine. However, the area of the glass must be sufficiently large compared with the volume of the chamber, so as to be able to evacuate heat sufficiently. Now, it is necessary to preserve a large degree of freedom of design of the headlight so as to be able to adapt to the internal structure of the vehicle and its body. This embodiment is therefore not entirely satisfactory. Its design is complex and restricting and increases the number of parts to be produced.